The hepatitis C virus (HCV) infects about 170 million people worldwide causing profound morbidity and mortality (McHutchison (2004) Am. J. Manag. Care 10:S21-S29). HCV is typically treated with the nucleoside analog ribavirin combined with one of several recombinant human alpha interferons. Though such treatments are effective, therapy is poorly tolerated, expensive, and not equally effective against all HCV genotypes (Manns, et al. (2001) Lancet 358:958-965). Better HCV treatments are therefore being modeled on other antivirals, which unlike interferon and ribavirin directly attack proteins that HCV synthesizes in human cells. Such “direct acting antivirals” (DAAs) typically are small molecules that inhibit viral enzymes, with the most common targets being viral polymerases and viral proteases. Two HCV protease inhibitors, telaprevir (Zeuzem, et al. (2011) N. Engl. J. Med. 364:2417-2428) and boceprevir (Bacon, et al. (2011) N. Engl. J. Med. 364:1207-1217), were recently approved for use in HCV patients, but neither alone eradicates HCV infection because HCV rapidly evolves to become resistant to the DAAs (Hiraga, et al. (2011) Hepatology doi: 10.1002/hep.24460). Protease inhibitors need to be administered with interferon and ribavirin, and as a consequence many patients still poorly tolerate the new therapies.
Telaprevir and boceprevir both inhibit the HCV nonstructural protein 3 (NS3). NS3 is one of ten proteins that are derived from the approximately 3,000 amino acid long polypeptide encoded by the HCV RNA genome. Viral and host proteases cleave the HCV polyprotein into mature structural (core, E1, E2) and non-structural proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B). The HCV nonstructural proteins form four enzymes. NS5B is a polymerase that synthesizes new viral RNA. The NS2 and NS3 proteins combine to form an autocatalytic protease. NS3 and NS4A combine to form a serine protease that cuts itself, cleaves the NS4B/NS5A, NS5A/NS5B junctions, and some cellular proteins. NS3 is also an ATP-fueled helicase that can separate and re-arrange RNA/RNA, RNA/DNA and DNA/DNA nucleic acid duplexes and displace nucleic acid bound proteins (Frick (2007) Curr. Issues Mol. Biol. 9:1-20).
Helicases have been widely studied as possible drug targets although progress has been slower than with other viral enzymes (Frick (2007) supra; Kwong, et al. (2005) Nat. Rev. Drug Discov. 4:845-853). Nevertheless, HCV needs a functional helicase to replicate in cells (Kolykhalov, et al. (2000) J. Virol. 74:2046-2051; Lam & Frick (2006) J. Virol. 80:404-4119; Mackintosh, et al. (2006) J. Biol. Chem. 281:3528-3535), and small molecules that inhibit HCV helicase catalyzed reactions also inhibit cellular HCV RNA replication (Paeshuyse, et al. (2008) Antimicrob. Agents Chemother. 52:3433-3437; Krawczyk, et al. (2009) Biol. Chem. 390:351-360; Stankiewicz-Drogon, et al. (2010) J. Med. Chem. 53:3117-3126). Therefore, NS3 helicase is a viable target for use in the treatment of HCV.